1. Field of the Invention
The present invention relates generally to power management in vehicles. The present invention relates more specifically to the control of an electric energy storage apparatus for electric, hybrid electric, and fuel cell vehicles.
2. Discussion of the Related Art
In recent years, environmental concerns and foreign oil dependency have increased our nation's interest in a viable alternative to standard internal combustion engines. Some promising alternatives include electric, hybrid electric and fuel cell vehicles. However, each of these systems is limited by the power supply that supplies power to the electric motor/generator. The following discussions are directed to hybrid electric vehicles, but one skilled in the art would recognize that this invention is also applicable in other electric vehicles, e.g., pure electric vehicles and fuel cells.
Hybrid electric vehicles (HEV) couple the power produced by an internal combustion engine (ICE) and an electric motor to propel the vehicle more efficiently than the ICE by itself. Fuel economy improvement is generally obtained by using a smaller ICE (set to provide the average vehicle power demand), augmented by the electric motor (provides power demand transients). The electric motor is powered by an energy source such a battery or an ultracapacitor. The energy source needs to store adequate energy to meet the averaged demand that is required from the electric motor under various driving conditions. In addition to the energy requirement, the source needs to be able to deliver short high-power charge and discharge pulses.
In principle, batteries have a relatively high energy density. However, they do not posses instantaneous charge and discharge capabilities. Further, if batteries are cycled at very high C-rates, the life of the pack is severely diminished, and may also lead to safety issues due to thermal runaway. Therefore, the battery packs in HEV's generally have to be oversized to ensure battery life and to avoid thermal runaway.
Due to their high specific power and near instantaneous charge and discharge capabilities, ultracapacitors have been considered for transient power supply and recovery in hybrid power trains. Therefore, an energy storage system which utilizes both a battery and an ultracapacitor can reduce the strain on the battery pack. The ultracapacitor absorbs and supplies the large current pulses, and the battery provides the average power demand. This, in turn, allows for the size of the battery pack to be reduced, and sized for the energy requirement of the cycle, rather than the power requirement.
Prior designs of the energy storage systems have various limitations. FIG. 1 shows an energy storage system with a direct parallel connection of the two sources. The energy storage system with the direct parallel connection of the two sources shows a battery 10 in parallel with an ultracapacitor 12 and connected to a buck-boost converter 14. And the output of the buck-boost converter 14 is connected to a controller 16. The controller 16 is connected to an electric motor/generator 18. This setup generally keeps the same voltage over both the battery 10 and the ultracapacitor 12, which in turn limits the power delivered form the ultracapacitor 12.
FIG. 2 shows a prior design of an energy storage system with a bi-directional DC/DC converter. A first buck-boost converter 22 placed between a battery 20 and an ultracapacitor 24. And a second buck-boost converter 26 placed between the ultracapacitor 24 and a controller 28 which is connected to an electric motor/generator 30. The output of the first buck-boost converter 22 is current controlled, and controls the current output out of the battery 20. The ultracapacitor 24 supplies the remaining power requirement to an electric motor/generator 30. This allows for the battery 20 voltage to be different than that of the ultracapacitor 24. It is beneficial to put the battery 20 on the input side of the first buck-boost converter 22 to be able to control the current output, and therefore the stress on the battery 20. This makes the ultracapacitor's 24 voltage the voltage that is supplied to the electric motor (bus voltage). Therefore, the bus voltage varies with the state of charge (SoC) of the ultracapacitor 24. Since the voltage of the ultracapacitor 24 can vary substantially, there is a large voltage swing on the input to the second buck-boost converter 26. Therefore, the second buck-boost converter 26 has to be stable for a wide voltage input range. At low ultracapacitor 24 voltages, input current of the second buck boost converter 26 can be very high leading to large internal resistance losses and a need for high rating switches. As such, this system is relatively inefficient and costly.
Another potential limitation of many electric and hybrid vehicles is a common chemical reaction inside of batteries. This chemical reaction is sulfation of negative plates due to high power partial state of charge operation. This is primarily a problem for lead-acid batteries. However, the formation of large non-reactive sulfate crystals in the negative plate can be prevented by battery conditioning. Battery conditioning is a complete charge with a slight overcharge of the battery which breaks up sulfate crystals and allows capacity of the battery to be regained.
Prior battery management systems that have previously been considered as a solution to premature end of life due to sulfation in electric vehicle applications have numerous shortcomings. One prior approach takes a single cell of a module off-line and conditions this single cell while the other cells in the module supply the load. This is not very efficient since there is additional weight in the battery pack that is not being used to power the vehicle. Also, the system is made more complex since each battery requires additional switches for conditioning needs. Another approach conditions the batteries while the vehicle is not in use by redistributing the charge in the batteries to different battery modules, and thus conditions the batteries. However, this can only be done off-line, which assumes that the system is able to assess when the user is not using the vehicle, and finish the charge redistribution before the battery is to be used again. The third approach uses the electrical grid for lead acid battery conditioning. This requires an interface between the battery and the charger and in some cases a special outlet that is able to handle the high power requirements.
There is a need for an improved energy management system. There is a need for an energy management system that allows for use of an ultracapacitor and a battery. There is also a need for an energy management system that efficiently conditions batteries.